This hack may not be everything that could replace a Tablet but sure is quiet near to be one and more evidently, fun.
You can use your Macbook TrackPad an use it as Touch panel that acts like a Tablet. As a result, you can draw sketch and Scribble your Masterpieces in Apps like Photoshop.
The app costs $25, free to try, needs Macbook with Snow Leopard. Though you can use fingers for the gestures, drawing and sketching works best with Inklet stylus. The gestures are all similar, not at all hard to adapt to. Watch the video to get a taste, then you can head on try a free demo.